So I have been told that back pressure is not a good thing on naturally aspirated motors and you should try to reduce all the back pressure as possible, then tune your car, and then you should get the best performance. Right?
I have a Saturn Astra with a 1.8l ecotech motor pushing 138hp. I don't plan on modifying the engine at all, so what would be the best route for reducing the back pressure to get the best performance?
Should I throw on an exhaust with 3-4" piping, and then have it properly tuned on a dyno?
I am really interested in how back pressure and engine performance works. So if someone can enlighten me, that would be great.
Backpressure is caused by restrictions in your exhaust. e.g. catalytic converters, mufflers, piping, bends in the piping etc. Basically youwant to have a free flowing exhaust after your headers/extractors.
What makes/limits the most power is the backpressure at the exhaust valve just prior to it opening to release the high pressure exhaust gasses in the combustion chamber. This is where headers/extractors come into play.
The length of the header harnesses the pressure wave from the exhaust pulse. As the exhaust valve "slams shut", a low pressure pulse runs down the header at the speed of sound. When it hits the collector, the low pressure "splits" and runs back up all the exhaust runners.
The length of the exhaust runners are "tuned" so that the low pressure pulse hits the exhaust valve just prior to opening (at particular RPM points). If you understand basic physics, the greater the pressure difference either side of a valve, the higher the flow (and scavenging - another topic).
My recommendations (in order):
1. Buy a high flow catalytic converter
2. Buy a set of headers designed for where you want most torque (4-1 will move torque higher and give more power at high RPM, 4-2-1 will give better midrange). Headers need to be equal length.
3. Buy >2" cat back. 3-4" is not necessary for small cid N/A
on a na motor you want a little bit of back pressure. if you just run open headers on a na motor you will loose quite a bit of low end. if you have a turbo car that is a different story, just run open downpipe for max hp tq
on a na motor you want a little bit of back pressure.
This is incorrect. Backpressure RESISTS the flow of air through the engine, and we all want good airflow through our engines, right?
If your statement was true, we would want a little restriction in the intake as well...
What you may be confused about is backpressure v. exhaust pulse tuning.
Would you say the loss of torque from running open headers MAY be due to the shorter exhaust having a higher resonant frequency instead of not having any backpressure?
Think about that and get back to me, because I might be full of shit...
Back pressure is the wrong term. You don't want any back pressure. What you are looking for is high exhaust velocity, which you can get by not having too big of an exhaust.
Back pressure on a NA 1.8l motor 
